1. Field of the Invention
The present invention relates to a semiconductor device contained inside a package thereof. Particularly, the present invention relates to a semiconductor device in which an acceleration sensor for detecting acceleration in three dimensions is contained inside the package.
2. Background Information
In recent years, acceleration sensors have come to be widely used in various fields of industry such as automobiles, robots, various precision equipments, etc. Particularly, the demand for semiconductor acceleration sensors using a MEMS (micro electro mechanical system) has dramatically increased, for it is small in size, light, inexpensive, and can be expected to operate accurately and reliably.
There is a type of semiconductor acceleration sensor which detects acceleration by using the piezoresistive effect, i.e., a phenomenon in which a resistance value changes in proportion to a generated stress. This type of semiconductor acceleration sensor usually has a structure in which a semiconductor chip that forms a portion of a sensor portion (hereinafter such semiconductor chip will be referred to as a sensor chip) is contained inside a package that is comprised of a ceramic material.
A sensor chip using the piezoresistive effect, for instance, has a structure in which a weight portion placed in the center thereof is supported by four flexible beams, each of which has one end fixed to a square fixing portion. These beams each have piezoresistive elements attached thereto, and a Wheatstone bridge circuit can be obtained by connecting these piezoresistive elements to each other via wiring patterns.
In a semiconductor acceleration sensor having such sensor chip, the beams will bend due to the stress generated by the inertial movement of the weight portion when there is a change in speed. At the same time, the piezoresistive elements attached to the beams will also bend. Due to such bending, the resistance value of each piezoresistive element will change, resulting in a change in the resistance balance in the Wheatstone bridge. Acceleration can be detected by measuring such change in resistance balance as a current change or a voltage change.
A package for housing the sensor chip is usually fixed to a predetermined circuit using an adhesive material such as a resin. Therefore, when the circuit substrate is deformed due to heat expansion, for instance, the sensor chip and the package will be deformed together.
Examples of methods used to deal with this type of problem are introduced in Laid-Open Japanese Patent Application No. 6-160423 (hereinafter to be referred to as patent reference 1) and Laid-Open Japanese Patent Application No. 6-289048 (hereinafter to be referred to as patent reference 2).
In patent reference 1, a plate-like spacer is arranged between a package including the sensor portion of a capacitance acceleration sensor and a circuit substrate, for the purpose of absorbing possible deformation generated at the circuit substrate and thereby preventing the package from becoming deformed.
In patent reference 2, only a portion of the lower surface of a package including the sensor portion of a capacitance acceleration sensor is fixed to a circuit substrate, and due to this arrangement, the amount of deformation transferred from the circuit substrate to the package will be reduced.
In these prior art methods, however, when heat is transmitted to the package from outside or from the circuit substrate, for instance, stress will be applied to the semiconductor chip serving as the sensor portion due to deformation generated in the package by means of the heat, or due to expansion in both the resin bonding the package and the sensor portion caused by the heat. This will lead to deformation of the semiconductor chip.
Particularly, with respect to a semiconductor acceleration sensor, in order to secure sufficient impact resistance, a silicone resin having a comparatively low elastic modulus is used in adhering the semiconductor chip serving as the sensor portion to the package. However, a resin having a comparatively low elastic modulus usually has a large heat expansion coefficient. Therefore, the semiconductor chip may be deformed to a considerable extent, depending on the heat transmitted to the resin.
When deformation is generated in the semiconductor chip serving as the sensor portion, the piezoresistive elements attached to the semiconductor chip will also be deformed. As a result, the electrical characteristics of the piezoresistive elements may change, which lead to the sensor chip not being able to detect acceleration accurately.
Therefore, the conventional semiconductor acceleration sensor has had difficulties in operating stably with respect to heat transmitted to the package thereof.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved semiconductor device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.